A circuit may vary the effective capacitance of a circuit element in response to a control signal. According to one example, a voltage-controlled oscillator (VCO) may vary the effective capacitance of an internal element in response to a signal that indicates a desired output frequency of the VCO. The output frequency then changes in response to the changed capacitance. In the case of a VCO, it is often difficult to vary an effective capacitance of the internal element to an extent required to produce a desired range of output frequencies. Some VCOs attempt to address this difficulty by switching in additional or alternative capacitive elements when needed. The additional or alternative capacitive elements may change a “center” frequency of the VCO, and may therefore increase the tuning range thereof.
The foregoing technique may cause a jump in the output frequency of the VCO. Accordingly, additional or alternative capacitive elements are usually coupled selectively to a VCO prior to operation of the VCO. This coupling may be based on a stored control word that indicates a desired center frequency of the VCO. The output frequency of the VCO is then fine-tuned during operation by varying the capacitance of a single circuit element.